When I was a kid I called them “coming attractions” or “the previews.” For me they were an incentive to make it to the movies on time. Before each film I’d watch the previews for films that wouldn’t arrive for months. Each weekend I’d go out knowing a movie could suck, but I I’d at least see one great preview.
Originally the advertisements were shown after the film. This is how they got the name “trailers.” Ironic, we now stay after the credits to watch the preview for the next Marvel franchise. As cool as these scenes are, people still leave during the credits, and this was a major issue in the early days of the trailer. Even though they’re no longer trailing the film, trailers kept the name and are now bigger than ever.
On VHS, previews could be viewed over and over and with DVDs and Blu-rays the process became even simpler. These days I consume most trailers from my mobile phone. On the Internet, trailers are the third most viewed video type behind news and user-created videos.
The idea of the trailer transcends film and is used to promote everything from video games to books and events.
This is from my site TrailerMadeTV.